Free radical polymerization of styrene monomer

ABSTRACT

High molecular weight polymers of vinyl aromatic monomers are prepared by free radical polymerization in the presence of 5 to 5000 ppm of a soluble organic acid having pKa from 0.5 to 2.5.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 07/651,072, filedFeb. 6, 1991, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a process for polymerization of vinylaromatic monomers. More particularly the present invention relates to animproved process for the free radical polymerization of vinyl aromaticmonomers to make high molecular weight polymers.

Currently, production of high molecular weight vinyl aromatic polymers,particularly polymers having weight average molecular weights (M_(w)) ofgreater than 300,000, is best performed by the use of anionicpolymerization techniques. This is due to the extremely slowpolymerization rates required to make high molecular weight vinylaromatic polymers using free radical chemistry. Disadvantageouslyhowever, anionic polymerization processes require expensive anionicinitiators and tend to produce discolored products due to the presenceof residual lithium-containing salts. In addition, anionic processesutilize different equipment than free radical processes. Consequentlycommercial producers of vinyl aromatic polymers by means of free radicalchemistry must invest in anionic polymerization equipment in order toprepare very high molecular weight polymers. Finally, anionicpolymerization cannot be employed to prepare many copolymeric products.In many cases the monomer is not amenable to anionic polymerization. Inother cases block copolymers are formed due to unequal reactivities ofthe comonomers.

It would be desirable if it were possible to produce high molecularweight polyvinyl aromatic resins utilizing free radical polymerizationequipment while obtaining rates that are commercially practical. Thus itis to the attainment of the preparation of such high molecular weightpolymers via free radical polymerization techniques that the presentinvention is directed.

According to the present invention there is provided a process for freeradical polymerization of a vinyl aromatic monomer to prepare a highmolecular weight polymer characterized in that the polymerization isconducted in the presence of from 5 to 5000 parts per million (ppm) of asoluble organic acid having a pKa from 0.5 to 2.5, at 25° C. It has beensurprisingly discovered that in the presence of such an amount of theseacids the free radical polymerization rate is substantially increasedthereby allowing the attainment of high molecular weight polymers inreasonable reaction times.

The vinyl aromatic monomers usefully employed according to the presentprocess include styrene, ring alkyl substituted styrene, particularlyC₁₋₄ alkyl and especially methyl ring substituted styrenes andα-methylstyrene. A preferred monomer is styrene. The polymerization canalso include a comonomer to prepare vinyl aromatic copolymers. Thecomonomer must be noninterfering with the acid. Examples include(meth)acrylonitrile, (meth)acrylic acid and C₁₋₄ alkyl esters thereof,N-C₁₋₄ alkyl maleimide, N-phenyl maleimide, maleic anhydride, etc. Inaddition the polymerization may be conducted in the presence ofpredissolved elastomer to prepare impact modified, grafted rubbercontaining products.

By the term "soluble" is meant that the acid is sufficiently soluble inthe reaction mixture to achieve the indicated concentration of organicacid. Preferred organic acids are miscible with neat styrene monomer.Suitable organic acids include the C₁₋₂₀ alkyl and aryl substitutedsulfonic and phosphonic acids. Examples include methane sulfonic acid,toluene sulfonic acid, camphorsulfonic acid, napthalene sulfonic acid,methyl phosphonic acid, phenyl phosphonic acid, etc. Strong acids, i.e.,organic acids having a pKa less than 0.5, are not desired due toincreased incidence of cationic polymerization as opposed to the desiredfree radical initiation. Preferred acids have pKa from 1.0 to 2.0. Apreferred organic acid is camphorsulfonic acid.

Similarly it has been discovered that at increased concentrations oforganic acid, cationic polymerization becomes prevalent. Generally,acids with higher pK, i.e. weaker acids, may be employed in higherconcentration without detrimental effect. Stronger acids are employed inrelatively smaller concentration. Cationic polymerization is undesirablebecause it results in extremely low molecular weight oligomer formation.Even small quantities of such low molecular weight product wouldsignificantly reduce the molecular weight average of the resultingproduct. Most preferred are amounts of organic acid from 50 to 5000 ppm.The amount of acid is measured with respect to the molar quantity ofvinyl aromatic monomer.

A free radical initiator may be employed to further improve the rate offree radical initiation. Suitable catalysts include organic peroxides orother well known free radical initiators.

The monomer may be polymerized in bulk, i.e., in the absence of adiluent, or in the presence of a diluent, i.e., in solution. Suitablediluents include toluene, ethylbenzene, and other noninterfering organicliquids. Preferably the reaction is conducted under bulk polymerizationconditions.

The polymerization rate according to the present process issubstantially increased and the resulting product has substantiallyincreased molecular weight compared to products prepared by free radicalpolymerization in the absence of an organic acid. However, because theproduct has increased molecular weight, the conversion rate is less athigher acid concentrations compared to lower acid concentrations. Thatis, the higher molecular weight polymers require longer reaction timesdespite incrementally faster polymerization rates. Preferred polymerproduct has a molecular weight (Mw) from 300,00 to 1,000,000, morepreferably 500,000 to 800,000, based on a polystyrene standard asmeasured by size exclusion chromatography.

The products are employed in applications where high molecular weightvinylaromatic polymers have previously found suitable uses. Particularlypreferred are molding polymers comprising the presently preparedpolymeric products. The product may be blended with other ingredientssuch as mold release additives, lubricants, colorants, ignitionresistant additives, impact modifiers, glass fibers, as well as otherresins such as polyvinylaromatic resins having different molecularweights, polyphenylene oxides, polycarbonates, elastomeric copolymerssuch as styrene-butadiene block copolymers, polybutadiene, etc.

Having described the invention the following examples are provided asfurther illustrative and are not to be construed as limiting.

EXAMPLE 1-3

Aliquots of styrene monomer which was purified by degassing andcontacting with alumina were placed in glass tubes. To each tube wasadded an amount of methane sulfonic acid further identified in Table 1.The tubes were sealed under vacuum and place in an oil bath at 150° C.for 1 hour. The tubes the weight average molecular weight of thepolystyrene in each tube measured using size exclusion chromatography.Results are contained in Table 1.

                  TABLE 1                                                         ______________________________________                                        Run       Methane Sulfonic Acid (ppm)                                                                      Mw                                               ______________________________________                                        *          0                 270,000                                          1         100                460,000                                          2         300                580,000                                          3         500                830,000                                          ______________________________________                                         *comparative                                                             

It may be seen that significant increase in molecular weight is observedupon addition of small quantities of the acid.

EXAMPLE 4-8

The reaction conditions of Examples 1-3 were substantially repeatedemploying various concentrations of camphorsulfonic acid (CSA) at areaction temperature of 140° C. Weight average molecular weight of theresulting polystyrene and conversion rates are provided in Table II.

                  TABLE II                                                        ______________________________________                                                               Conversion                                             Run    Amt. CSA ppm    Rate (%/hr)                                                                              Mw                                          ______________________________________                                        *       0              43         280000                                      4      100             25         400000                                      5      250             18         510000                                      6      500             14         650000                                      7      750             12         700000                                      8      1000            10         710000                                      ______________________________________                                         *comparative                                                             

EXAMPLES 9-10

The reaction conditions of Examples 1-3 are substantially repeatedutilizing phenylphosphonic acid (PPA) at a reaction temperature of 150°C. Weight average molecular weight of the resulting polystyrene andconversion rates are provided in Table III.

                  TABLE III                                                       ______________________________________                                                Amt. PPa      Conversion                                              Run     (wt. %)       Rate (%/hr)                                                                              Mw                                           ______________________________________                                        *       0             50         240000                                       9       0.1           40         280000                                       10      0.5           25         430000                                       ______________________________________                                         *comparative                                                             

What is claimed is:
 1. A process for free radical polymerization ofstyrene to prepare high molecular weight polystyrene comprisingconducting the polymerization in the presence of from 5 to 5000 ppm of asoluble organic acid having pKa from 0.5 to 2.5 at 25° C.
 2. A processaccording to claim 1 wherein the organic acid is methane-sulfonic acid.3. A process according to claim 1 wherein the organic acid iscamphorsulfonic acid.
 4. A process according to claim 1 wherein thepolystyrene has a Mw from 300,000 to 1,000,000.